As semiconductor and integrated circuit technology has advanced, there has been a trend toward high-functionality integrated circuit components with numerous input and output pads, together with a demand for reduced chip size, weight, and power consumption. Consequently, as integrated circuits are reduced in size, they increasingly have smaller output pads arranged more closely together than ever before.
To match these high functionality integrated circuits, there is a demand for printed wiring boards consisting of closely arranged pads for solder attach of the integrated circuit. However, the miniaturization of the spacing between integrated circuit pads is currently happening at a greater rate than the miniaturization of solder pads on printed circuit boards, in some applications. In addition, the ability to protect these closely spaced pads with solder masks has not kept up with the reduction in space between pads on the integrated circuit. Without solder mask protection between closely spaced adjacent pads, there exists an increased risk for bridging and electrical shorting during the solder reflow process utilized in attaching integrated circuits to printed wiring boards. Consequently, there is an interconnection technology gap for some modern devices.
To make such devices function, printed wiring boards may have extra routing layers to handle the pads of the integrated circuits, or utilize fan-out packaging. This results in the package size of an integrated circuit being larger than the integrated circuit itself, which may limit system miniaturization. In addition to these desires for miniaturized devices, it is also desirable in some cases to construct these devices from a flexible, and not rigid, substrate.
A prior art miniaturized device is described in U.S. Pat. Pub. 2007/0025092 to Lee et al. This reference discloses an electronic device comprising a plurality of build-up layers defining circuit interconnections and that comprise one or more thin film type of embedded passive devices, at least a cavity formed in the build-up layers. An active device is disposed in the cavity and electrically connected to the circuit interconnections of the build-up layers. The packages have a chip reworkability, an easier thermal management, and a relatively very thin profile. However, these packages are not constructed from flexible materials, and may therefore be unsuitable in some situations. In addition, even thinner packages may be desirable.
One material now being used as a substrate from which to construct thin and flexible printed wiring boards is biaxially-oriented liquid crystal polymer (LOP). The molecules in LCPs have rigid, rod-like shapes, and maintain a crystalline order when in a liquid phase or when heated and melted. The Processing and Assembly of Liquid Crystalline Polymer Printed Circuits, T. Zhang, W. Johnson, B. Farrell, and M. St. Lawrence, “The processing and assembly of liquid crystalline polymer printed circuits,” 2002 Int. Symp. on Microelectronics, 2002. discusses the construction of a printed circuit board using LCP as a substrate. A photoresist is first applied to a copper clad laminate, exposed, and developed to define a desired circuit pattern. The actual circuit is then defined by etching the exposed copper away. Multiple circuit layers are laminated together in a hot press or autoclave to form a multilayer. Holes or vias are created in the substrate via mechanical or laser drilling. A desmearing step is then performed to remove debris from the vias or holes, thereby preparing the LCP material for metal deposition. A metallization step is next performed, and a conventional solder mask is applied to the LCP substrate. Solder is then applied through the conventional solder mask to complete the construction of the LCP printed circuit board.
While this design does allow for the creation of thin, flexible printed circuit boards, it still suffers from the same drawbacks as described above with respect to the attachment of integrated circuits with closely spaced pads thereto. As such, additional methods of connecting integrated circuits to printed circuit boards are needed.